Electrochemical capacitors are a class of high-rate energy storage/discharge devices which use electrolytes and electrodes of various kinds. Electrochemical capacitors, like batteries, are essentially energy storage devices. However, unlike batteries, they rely on charge accumulation at the electrode/electrolyte interface to store energy. Charge storage in electrochemical capacitors therefore is a surface phenomenon. Conversely, charge storage in batteries is a bulk phenomenon occurring within the bulk of the electrode material. As a result of the differences in the charge/discharge mechanism, and selection of electrode materials between capacitors and batteries, the discharge profiles and discharge rates of the two devices are radically different.
For most battery systems voltage discharge profiles are typically flat for the major portion of the discharge cycle. Once a predetermined end-of-life voltage is reached, the voltage profile drops abruptly to zero. An example of a typical battery discharge voltage profile of the prior art is illustrated in FIG. 1, wherein discharge voltage is illustrated on the ordinate, and discharge time is illustrated on the abscissa, and which shows that the discharge voltage, line 12, remains substantially constant for a substantial period of the overall discharge cycle, dropping off rapidly to zero near the end of the discharge cycle. The advantage of the profile illustrated in FIG. 1 is that it is able to deliver a constant voltage for a prolonged period of time.
Conversely, capacitors such as conventional electrolytic and/or double layer capacitors, have the capability to deliver their stored energy very rapidly, i.e., in less than one second. This capability is necessary for delivering the brief bursts of energy required of certain applications, such as when a portable radio or cellular telephone is transmitting. The discharge rate is known as the "C" rate, and is an industry standard for stored energy discharge speed. A 1.0 C rate refers to the ability of a device to discharge its stored energy in one hour. A 10 C rate refers to a device capable of discharging its stored energy into a load in 6 minutes, and a 100 C rate device discharges its stored energy into a load in 0.6 minutes.
Unfortunately, while the discharge is very rapid, the discharge profile is linearly decreasing with time. Hence the highest discharge voltage occurs at the moment discharge starts, and degrades rapidly from there. The voltage discharge profile of a capacitor device of the prior art is illustrated in FIG. 2 wherein voltage is illustrated on the ordinate, and time is illustrated on the abscissa, and wherein the discharge profile is illustrated by line 14. As may be appreciated from FIG. 2, the discharge voltage is linearly decreasing with time. Hence, if a particular voltage is required, such as a voltage in excess of 19 volts, only a small fraction of the discharge cycle is actually used by the host device. Thus, only the stored energy defined by block 16 is usable to the host device; the rest remains stored and is inaccessible.
Accordingly, there exists a need for an electrochemical device capable of delivering a constant voltage discharge for a substantial portion of the discharge cycle. Moreover, in order to address the high pulse power requirements of many host devices, the total discharge cycle should be extremely fast, i.e., on the order of or in excess of a 100 C rate.